Polycarbonate resin is being used in various uses in large quantity owing to its self-extinguishing nature, excellent optical characteristics, electrical characteristics and dimensional stability, good mechanical properties such as impact resistance and high heat-resistance, transparency, etc.
Known processes for the production of polycarbonate resin include the direct reaction of an aromatic dihydroxy compound such as bisphenol A with phosgene (interfacial polymerization process) and the transesterification of an aromatic dihydroxy compound such as bisphenol A with a carbonic acid diester such as diphenyl carbonate in molten state (melt process). Among the above processes, the transesterification of an aromatic dihydroxy compound with a carbonic acid diester (melt process) is expected to be promising in future since the process is free from the problem of using toxic phosgene and a halogen compound such as methylene chloride as a solvent in contrast to the interfacial polymerization process and a polycarbonate can be produced at a low cost by the process.
A transesterification catalyst is used usually in the melt process for the production of a polycarbonate by transesterification reaction to improve the production efficiency ("Plastic Material Course 17, Polycarbonate", pp.48-53, The Nikkan Kogyo Shimbun, Ltd., Toshihisa Tachikawa, Shoichi Sakajiri).
The preferable transesterification catalyst system is a combination of a basic nitrogen compound (also called a nitrogen-containing basic compound) and/or a basic phosphorus compound with an alkali metal compound which produces, in high productivity, a polycarbonate having good physical properties such as color tone, containing little branched structure in the polymer molecule, having good physical properties such as melt-fluidity and containing limited amount of foreign materials such as gel in the produced polycarbonate resin.
However, the polycarbonate produced by melt-polymerization process has a stability problem since the polymer contains alkali metal compounds and other metallic compounds used as a transesterification catalyst.
For example, there are short-term stability problems such as discoloration, lowering of molecular weight and generation of black foreign materials in melt-molding under heating. Long-term problems such as discoloration and lowering of molecular weight in a hot environment as well as gradual lowering of mechanical properties of molded articles are also observed. Especially, it has problems such as susceptibility to hydrolysis of the molecular chain when its molded article is kept under a special condition, for example in water, especially in hot water or in steam atmosphere.
For solving these problems, specifications of JP-A 4-328124 (hereunder, JP-A means "Japanese Unexamined Patent Publication") and JP-A 4-328156 disclosed a process for stabilizing a polycarbonate by deactivating the transesterification catalyst with an acidic compound comprising a sulfonic acid ester.
Further, the above problems have been remarkably relieved by JP-A 8-59975 which proposes the use of a specific sulfonic acid derivative as a catalyst deactivation agent (catalyst inactivation agent) to deactivate an alkali metal compound and/or an alkaline earth metal compound used as a component of a transesterification catalyst.
It is already known in general that various stabilizers are usable for preventing the short-term (especially in molding) or long-term discoloration, molecular weight lowering, etc., of a thermoplastic resin. It is also known that single or combined use of various phosphorous acid ester compounds, epoxy compounds and hindered phenol compounds as a stabilizer is effective for the stabilization of aromatic polycarbonates.
However, the desired stabilization effect is difficult to attain even by using these stabilizers according to conventional formulations to an aromatic polycarbonate produced by using a transesterification catalyst containing metallic element. The difficulty was assumed to be caused by impurities in the polycarbonate e.g. in JP-B 7-5828 (Asahi Chemical Industry Co., Ltd.) (hereunder, JP-B means "Japanese Examined Patent Publication"), which proposed a process for the stabilization of an aromatic polycarbonate by using (1) a phosphorous acid mono or diester in combination with (2) a hindered phenol antioxidation agent or a phosphorous acid triester in a polymer having an ionic chlorine content of 0.5 ppm or below.
However, the stabilizer proposed by the above patent application is not effective, as shown in the Comparative Example 3 of the application, on a melt-polymerized polycarbonate produced by using bisphenol A sodium salt as a transeterification catalyst in an amount of 5 ppm in terms of sodium component based on the bisphenol A.
In contrast to the above method, the polycarbonate stabilization effect is by far remarkable in the case of using a catalyst deactivation agent (catalyst inactivation agent) proposed by the inventors of the present invention in JP-A 8-59975, especially a sulfonic acid phosphonium salt.
With single use of an alkali metal compound catalyst as a transesterification catalyst, e.g. in the transesterification reaction of diphenyl carbonate (hereunder abbreviated as DPC) with bisphenol A (hereunder abbreviated as BPA), the reaction rate is too slow to be allowable from an industrial viewpoint when the transesterification conversion ratio is smaller than 50%, that is, at a stage in which a large amount of DPC having relatively low boiling point existing in the system makes it impossible to raise the reaction temperature. Accordingly, it is necessary to use the alkali metal compound in an amount larger than 10.times.10.sup.-6 chemical equivalent based on 1 mol of BPA to achieve an industrially significant reaction rate.
If such a large amount of an alkali metal compound catalyst is used, a branching component is liable to be generated by the transfer of polycarbonate bonding when the polymerization temperature is increased at the later stage of the reaction. Accordingly, a melt-polymerized polycarbonate produced by transesterification has poor fluidity compared with a polycarbonate produced by interfacial polymerization owing to the presence of a larger amount of such branching components.
It is necessary to keep the amount of the above branching component at 0.2 mol % or below based on the repeating unit for producing a melt-polymerized polycarbonate having a fluidity comparable to that of a polycarbonate produced by interfacial polymerization. For satisfying the above condition, the amount of the alkali metal compound catalyst is required to be limited to 10.times.10.sup.-6 chemical equivalent or below, more preferably 5.0.times.10.sup.-6 chemical equivalent or below, based on 1 mol of BPA.
Effective for suppressing the branch formation and keeping the transesterification reaction rate at an industrially advantageous level is a process in which the alkali metal compound catalyst is applied in an amount limited to 5.0.times.10.sup.-6 chemical equivalent/1 mol of BPA or below and combined with a basic nitrogen compound catalyst in an amount of 1.times.10.sup.-5 chemical equivalent to 5.0.times.10.sup.-4 chemical equivalent in terms of basic nitrogen based on 1 mol of BPA.
The stability of a polycarbonate is maintained at an extremely desirable level as shown in the above invention by applying to the polycarbonate produced by the above method, a sulfonic acid phosphonium salt singly or in combination with a phosphorous acid ester compound or a phenolic antioxidant according to the process disclosed in JP-A 8-59975.
Improved stability under a high-temperature condition as mentioned above, in other words under an accelerated condition, has been attained to a certain extent by adopting the above-mentioned various proposals.
However, it has been found that gradual yellowing of chips and molded articles takes place when the chips or molded articles are stored over a long period in an open air, which is under room temperature, a low temperature condition distinct from the above high-temperature condition. The tendency is especially remarkable if the chips are stored in a bright place.